Extract an interface for ServiceConfigAccessor

This is a simple refactoring before later commits where it will be
beneficial to have an interface for testing.

This mirrors the situation in com.android.server.timezonedetector and is
part of a sequence of changes to make com.android.server.timedetector
follow the patterns for configuration used there.

Bug: 172891783
Bug: 229740080
Test: Build only
Change-Id: If2cb5468c5899b85cfe31522ebdba8517e9e359a